1. Field of the Invention
The present invention relates to synchronizing a receiver and a transmitter in digital transmission systems.
2. Description of Prior Art
In digital transmission systems, the information bits to be transmitted are combined into information blocks, each information block corresponding to a frame structure. Each information block includes, in addition to the information bits, a number of additional information bits necessary for effecting the data transmission.
Since the information sent out can be detected with a receiver, in addition to retrieving a sampling clock, it is also necessary to determine the point in time at which point a respective information block is received. In other words, a block synchronization must be conducted in the receiver so as to synchronize the receiver and the transmitter relative to each other.
In order to achieve synchronization, a transmitter sends out, for example, a synchronization signal known to the receiver. In the receiver, a receive signal, which includes the synchronization signal, is processed to detect a position of the synchronization signal in the receive signal, and/or to detect a time of occurrence of the synchronization signal. To this end, a cross-correlation may be conducted, for example, between the receive signal and a copy of the synchronization signal so as to find the synchronization signal in the receive signal. Since it is necessary, for accurately determining a position, to send out a broad-band synchronization signal, determining a position is associated with considerable computing complexity, since it is always required to process broad-band signals at a high sampling clock.
If, for example, a very broad-band digital signal (RF bandwidth B) is to be synchronized, i.e. if either fixed, known synchronization signals (preambles) are to be found in a continuous receive signal, or if fixed, known preambles are to be found within signal sections (signal bursts), the receive signal must be sampled and processed, on the grounds of the sampling theorem, at least with a bandwidth B so as to find the synchronization signals. If the transmitter or the receiver have just been switched on, there is as yet no information whatsoever about where the synchronization signal (preamble) might be, for example.
For this reason, the entire receive signal must be sampled and processed at the high bandwidth B over a relatively long period. If sampling is effected, for example, at a bandwidth of 77 MHz to 100 MHz, 100 million tests per second will have to be performed with regard to the preamble start, i.e. with regard to a start of the preamble in the receive signal, in order to find the preamble without prior information. If one cross-correlation, for example, is conducted in each test, each test involves a very large number of computing operations which require considerable system resources. A direct or blind (on-line) search, for example, for a preamble in a broad-band signal is impossible due to this high expenditure.